David Lennon's research involves applying a variety of spectroscopic techniques to probe the interaction of atoms and molecules with catalyst surfaces. Over the last two decades he has studied processes as varied as methane reforming to produce syngas (CO + H2), Fischer-Tropsch synthesis to generate fuel and chemicals, the production of methyl chloride (an intermediate in polydimethylsiloxanes), understanding the formation, and the mitigation, of by-products in isocyanate synthesis (the monomers for polyurethane manufacture) and selective hydrogenation for fine chemical synthesis.
He has made a major contribution to the study of methanol-to-hydrocarbons (MTH) reaction, a reaction first commercialised in the 1970s. The reaction uses a zeolite catalyst, and while there is general agreement that the pores of the zeolite act as a microreactor, key details of what is happening inside the zeolite and how it deactivates are still debated.
The process reacts methanol, which is widely available from a variety of sources, including biomass, to a mixture of low molecular weight alkenes (mainly ethene, propene and butenes) and methylated aromatic molecules, i.e. gasoline. David's group used neutron scattering to observe the “vibrational fingerprint" of the hydrocarbon pool for the first time – that is, to see what is present in an active catalyst as it reacts. It also provided insight into the nature of the carbon that causes the catalyst to deactivate, which was surprisingly well-structured, resembling glassy carbon.
This project has produced fundamental insights into the process with direct industrial relevance, as demonstrated by two EPSRC industrial CASE awards provided by global science and chemicals company, Johnson Matthey, who also provided the catalysts and analytical characterisation.
David's group also applied neutron scattering to investigate how the same catalyst can be used to crack long chain alkenes to propene (propylene). This is a valuable commodity chemical that is the monomer for the vast range of products made from polypropylene. This showed that the MTH and the alkene cracking reactions are strongly related, and both go by similar mechanisms, which had not been generally recognised. The work also studied the same catalyst after it had been steam de-aluminated – a process where steam at ~700 °C is passed through a zeolite, greatly reducing the number of active sites by removing the aluminium. The resulting material is much more like that used in working industrial reactors and the reactivity is correspondingly modified. Surprisingly, these materials have been little studied academically and this new understanding helps explain why this is the material of choice industrially.
In addition to the specific knowledge gained about this reaction, this project has raised awareness within Johnson Matthey of the capabilities of neutron scattering and contributed to the creation of the Johnson Matthey - ISIS fellowship, which is about to be renewed for a further three years.
Separately, the wider ISIS user community have benefitted from a collaboration between David's group, the ISIS Pressure and Furnace section and the Molecular Spectroscopy Group. The Glasgow/ISIS catalysis rig has been developed over a number of years to prepare the large catalyst samples needed for neutron scattering (typically these are 100-1000 larger than used in conventional micro-reactor lab-based studies). Recent major improvements to the rig include on-line quantitative analysis by gas chromatography and the ability to handle liquid products. The rig is heavily used by a variety of academic and industrial users and the recently enhanced capabilities are already popular with the user community.
Evidence of impact
This particular project has been highly productive: both students have successfully submitted their PhD theses, 13 publications and at least a dozen presentations have resulted. David is the author of over 55 published ISIS papers, and he has supervised 14 students whose PhD projects have involved the use of neutrons. Commencing in 2021, he also has two students sponsored by Johnson Matthey whose projects will involve the use of neutrons.